Can't imagine this being anything else than a tumble wheel. Probably a misnomer or a typo (even if it does twice). And tumbling and stumbling are somewhat related, too...

http://www.google.com/patents/US4056200
Abstract
A high speed stacker for grouping small tablets or similar products utilizes a vertically oriented tumble wheel having peripheral pockets for carrying the products serially between a receiving point and a discharge point. At the discharge point, the products are unloaded onto a stacking tray by means of a pair of stacking wheels on opposite axial sides of the tumble wheel. The stacking wheels are driven rotatably in opposite directions about axes lying in a plane generally tangent to the tumble wheel at the discharge point. The stacking tray immediately below the wheels receives the discharged products in series from the stacking wheels and holds them in a stack on a support surface of the tray.
(…)
Description
BACKGROUND OF THE INVENTION
The present invention relates to product handling machinery such as packaging machinery and relates more particularly to stackers such as used in packaging machinery for grouping products together prior to envelopment in a wrapper.


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Note added at 6 hrs (2013-05-29 21:12:23 GMT)
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Erratum: even if it does ***occur*** twice

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Note added at 19 days (2013-06-17 15:49:14 GMT) Post-grading
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@Asker: Graag gedaan Sanmar!